The present invention relates generally to the field of implantable articles. More particularly, the invention relates to bone prostheses and casting processes for manufacturing the same.
Joint replacement surgery is quite common and enables many individuals to function normally when otherwise it would not be possible to do so. Artificial joints are normally composed of metallic and/or ceramic components that are fixed to existing bone.
Artificial hip joints, for example, include several components. A femoral component of an artificial hip comprises an elongate stem or shaft at its distal end that is affixed within the medullary canal of the femur. A proximal end of the stem includes a neck region, to which is attached a femoral head. The acetabular shell is a separate component of an artificial hip joint that is affixed within existing bone such as the acetabulum. The acetabular shell includes a cup-like region that receives the femoral head. The femoral head and the acetabular shell form an articulation couple and smooth low frictional movement of the femoral head within the shell is essential to ensure proper functioning of the artificial hip joint.
Metal to metal articulation couples are often used in artificial joint construction and at least one of the articulation members is often fabricated from a low friction polymeric material. A common low friction polymeric coating material that is utilized as an articulation surface of an articulation member is ultrahigh molecular weight polyethylene (UHMWPE). UHMWPE is a durable polymer that has a very low coefficient of friction and enables smooth movement of the two components over each other.
Natural friction within a replaced, artificial joint can cause sub-micron particles of debris (e.g., metal from the joint or polymeric liner material) to become dislodged and to migrate within the joint. The phenomenon of wear debris within artificial joints is a serious problem that can inhibit the proper mechanical functioning of the joint. Wear debris can also lead to osteolysis and bone deterioration. If wear debris develops within an artificial joint it must usually be corrected by surgical removal of the debris or by subsequent replacement of the artificial joint.
The articulation couple of artificial joints is believed to be the principal source of wear debris. Currently, the state-of-the-art articulation couple in an artificial hip joint, for example, is a cobalt-chromium femoral head seated within a cobalt-chromium acetabular shell that is lined with UHMWPE. Despite this being one of the more advanced articulation couples, significant wear debris is believed to result from erosion of the polyethylene liner material.
A polymeric liner material is generally necessary to reduce friction in the joint, and these liner materials often contribute to wear debris. Although much research is focused on metal/metal articulations, current technology has not permitted the widespread use of metal/metal articulations.
Ceramic/ceramic articulation couples are believed to be potentially useful in artificial joints. Their actual use in artificial joint is, however, very limited because of the inherently low tensile strength of these materials. Ceramic materials are also quite brittle and their use as orthopedic implants, in load bearing applications, is further limited due to the risk of unpredictable catastrophic failure. See, Cooke, Clinical Orthopaedics and Related Research, 276:135-146 (1992). The practical use of ceramic/ceramic articulation couples also poses additional challenges with respect to fixation within bone and the manufacture of joint components having complex shapes.
Another limitation of previously known metal/metal articulations, ceramic/ceramic articulations, and metal/ceramic articulations is that there is little dampening of forces conveyed to the joint. This can place excessive loads on the joint prostheses, eventually leading to weakening or failure of the prostheses. Polymeric liners, used on the wear surface of the acetabular shell, function to some extent as a dampening mechanism by absorbing a portion of the load forces communicated to the hip joint. However, high loads and normal wear in the joint can, over time, contribute to the formation of wear debris in the joint.
Accordingly, there is a need to develop artificial joint constructions that utilize articulation couples that are improved so as to combine excellent frictional and load absorbing properties with less susceptibility to developing wear debris.
It is thus an object of the invention to provide a hip joint prosthesis that has a strong and effective low friction articulation couple. Another object is to provide an artificial joint articulation couple that greatly reduces or eliminates the tendency for wear debris to develop within the joint. It is also an object to provide a hip joint prosthesis that exhibits relatively high strength and which is effective to reduce loading forces applied to the prosthesis. A further object is to provide effective manufacturing techniques for such a hip joint prosthesis. These and other objects will be evident from the drawings and description that follow.